1. Field of the Invention
The present invention relates to magnetic resonance type isolators and in particular, relates to magnetic resonance type isolators that are, for example, used in a microwave frequency band.
2. Description of the Related Art
Typically, isolators have a characteristic of only transmitting signals in a specific direction and not transmitting signals in the opposite direction. Isolators are included in transmission circuit units of mobile communication devices such as cellular phones. Known examples of magnetic resonance type isolators include those described in Japanese Unexamined Patent Application Publication Nos. 63-260201 and 2001-326504. Magnetic resonance type isolators utilize a phenomenon that occurs in which, when high-frequency currents that have the same amplitude but differ in phase by about ¼ of a wavelength flow through two orthogonal lines (having four ports), a magnetic field (circularly polarized wave) is generated at the intersection of the two lines, and the circulation direction of the circularly polarized wave is reversed in accordance with the progression directions of the electromagnetic waves of the two lines. That is, a ferrite is arranged at an intersection of two lines and a static magnetic field is applied, which is necessary for magnetic resonance, by using a permanent magnet, and accordingly a positively circularly polarized wave or a negatively circularly polarized wave is generated by a wave being reflected from a sub-line in accordance with the propagation direction of an electromagnetic wave propagating along a main line. If a positively circularly polarized wave is generated, a signal is absorbed by the magnetic resonance of the ferrite, whereas if a negatively circularly polarized wave is generated, magnetic resonance does not occur and the signal passes through. A reactance element, which causes a signal to be reflected, is connected to an end portion of the sub-line.
However, to date, magnetic resonance type isolators have had a main line having a length of about ¼ of a wavelength so that the main line would resonate and have included two reactance elements, and consequently have had a large size of, for example, 20 mm by 20 mm for a frequency of about 2 GHz. This is not compatible with the current situation in which mobile communication devices have been becoming increasingly smaller in recent years and the density with which components thereof are mounted has been becoming increasingly high. Furthermore, in the case where a power amplifier is connected to the input side, it is preferable that the impedance on the input side be low and it is preferable that the impedance on the output side be higher than that on the input side. However, to date, in magnetic resonance type isolators, these requirements have not been satisfied and it has been necessary to provide an additional impedance conversion device as a separate component.